This invention relates to an off-peak, electrically heated, heat storage system, particularly for stored heating plants. This system includes a storage container to accommodate the salt used as a heat accumulator medium, an evaporator to generate steam, a heat-exchanger and a condenser installed in series.
Heat storage systems or accumulators are known and are used for utilizing cheap off-peak electrical energy for the production of heat which is required at a later time. Various types of construction have been used for storage heaters which have, however, not been completely successful in practice. Problems have arisen particularly with heat accumulators which use fusible salts as the accumulator medium.
Fusible, that is to say meltable, salts are particularly useful as the accumulator medium; for the heat accumulation is due not only to the specific heat, but also to the latent heat of fusion. Thus more heat can be accumulated for the same volume of the storage medium. Heat accumulators of this type, therefore, require only a relatively small space.
The repeated cycling between solid and liquid states during operation causes difficulties. Due to the different expansion coefficients of the container material and the solidified salt, stresses can occur to such an extent that the container can be destroyed. A further difficulty is the even extraction of heat from the heated salt. Whilst the heat-transfer from the liquid salt to the evaporator-system is very good, the heat-transfer from the solidified salt is not, because of the poor thermal conductivity of the salt. These problems can only be solved by an appropriate design of the evaporator system.
In the West-German Patent Specification No. 2,039,586 a heat accumulator is described which uses a fusible salt as the accumulator medium. The salt is stored in the container and is heated directly by heating elements immersed in the salt. A cylindrical evaporator also is located in the container, into which water is injected to produce steam. The water is fed to the evaporator through a pipe which is immersed in the lower part of the evaporator. Because of the heat extracted by the evaporating water the salt solidifies on the bottom of the container first.
The cavities, which result from the shrinking of the solid salt are filled from above by liquid salt, until the total salt content is completely solidified. Upon heating the compacted salt-core thus formed on the bottom of the container, stresses occur due to expansion of the solid salt, which lead to the deformation of the container casing.
Further, it is known that in air-conditioned buildings in summer the energy requirements for the refrigeration plants are higher than those in winter for heating. Cool air for buildings has to be produced during the day when a high demand for electricity exists. This load, which is above the output capacity of the power stations has resulted in some cases in the breakdown of the power supply. It is therefore very desirable to use off-peak electricity also for refrigeration.
Attempts have been made to solve this problem by special cold storage units. In these, water is used as a storage medium, which is cooled during the night and is used during the day in the cooling plant. Due to the low effective temperature gradient these storage units are large and therefore costly to construct. Further, it is a disadvantage that a cold storage unit is required in addition to a heat storage unit.